Feline leukemia virus (FeLV)-induced lymphosarcoma in cats is a unique model to study the immune mechanisms involved in the initiation and progression or dimunition of neoplastic cell growth. We have shown that FeLV-infected cats with lymphosarcoma are hypocomplementemic and that their sera contain high levels of circulating immune complexes (CICs). Our studies indicate that the complexes contain FeLV and IgG. Both FeLV and FeLV-containing complexes were shown to activate feline complement (C) in vitro. However, feline C is inhibitory for C-mediated cell lysis including intermediate sheep cells, FeLV, and a leukemic feline cell line, FeLV-74. More recently, in in vivo studies of an extracorporeal immunosorption procedure using Staphylococcus aureus or purified Staph protein A bound to filters to treat leukemic cats, we have observed a dramatic remission of the tumor and clinical improvement in the treated animals. Complete remission is observed in more than 90% of aleukemic leukemic cats and a partial remission in cats that normally do not recover. The remission was associated with increasing levels of feline gamma-interferon, C-mediated cytotoxic antibody levels, drop in FeLV antigen (gp70) using a monoclonal antibody prepared in our laboratory, and a drop in CICs. We propose to isolate and purify cat interferon, cytotoxic C-dependent antibody, and study the mechanisms in depth both in vivo and in vitro of the role of antibody in the destruction of tumor cells. In order to rule out any leaching of Protein A from the columns, we have also begun to inject purified Protein A intravenously using leukemic cats. We will include the above parameters in sera of these cats as well. (HF)